This invention relates to a decanter centrifuge. More specifically, this invention relates to a decanter centrifuge with structure for controlling the rate of cake discharge to thereby control cake moisture content. This invention also relates to an associated method for operating a decanter centrifuge.
A decanter centrifuge generally includes an outer bowl, an inner hub carrying a worm conveyor, a feed arrangement for slurry to be processed, and discharge ports for cake solids and clarified liquid. The bowl includes a cylindrical section and a conical beach section. The bowl and the hub are rotated at high, slightly different angular speeds so that heavier solid particles of a slurry introduced into the bowl are forced by centrifugation to settle into a layer of sediment along the inner surface thereof. By differential rotation of the worm conveyor and the bowl, the sediment is pushed or scrolled to a cake discharge opening at the smaller, conical end of the bowl. Additional discharge openings are provided in the bowl, usually at an end opposite of the conical section for discharging a liquid phase separated from the solid particles in the centrifuge.
One of the goals in centrifuge operation is to produce cakes with a low moisture content. One proposed method, published in Research Disclosure, March 1993, Number 347, for reducing cake moisture content entails the disposition of a flow control structure proximate to the cake discharge port to reduce the volume flow rate of the cake by 25% to 75%. The flow control structure could be a ring shaped darn extending radially outwardly from the axis of the bowl, a dam disposed between two turns or wraps of the conveyor, an increased beach climb angle, an increased conveyor blade thickness, or an increased or decreased conveyor helix angle. It was asserted that by decreasing the volume flow rate of the solids by about one-half, or between 25% and 75%, the velocity at the interface between the liquids and the sedimented solids is in the reverse direction, i.e., towards the pool and away from the cake discharge port. Liquid from the pool and liquid expressed from the cake layer are drained back into the pool rather than carried out of the bowl with the sedimented solids. This is typical of a fluid-like cake.
For a cake which has a consistency such that it behaves more like a solid or granular solid as opposed to a fluid, the additional resistance imposed at the cake discharge end of the centrifuge bowl results in a thicker cake layer there as well as generally along the entire beach. A thicker cake leads to a higher compaction pressure under centrifugal force. Also, additional re-circulation of the cake in the beach is deemed possible which results in longer cake retention time before discharge of the cake from the centrifuge. Both higher compaction pressure and longer retention time would effect better liquid expression from the cake, resulting in drier cake.
In consequence, with proper control of cake flow rate, drier cake can be obtained irrespectively of the nature of the cake, whether it be a solid-like or fluid-like cake.
It is also known to form a dip weir along the outer surface of the conveyor hub, at or about the location of the junction between the cylindrical and conical sections of the bowl, to serve in selecting the driest portion of the cake at the discharge end of the bowl. The dip weir blocks the transport of the sludge cake in such a manner that the most compacted part of the cake, adjacent to the inner bowl surface, passes under the dip weir and reaches the cake discharge opening. In conventional practice, the dip weir is fixed to the hub so that the radial gap between the outer edge of the dip weir and the inner surface of the bowl is constant or fixed. The designer must position and dimension the weir to minimize cake moisture content while not increasing cake transport resistance through the gap so as to unduly limit the solids capacity of the machine. The optimal gap height depends on the nature of the cake, the G-level, and the cake flow rate or solids throughput. The designer is forced to guess at the appropriate gap height, guided somewhat by past experience. If the gap height is guessed incorrectly owing to variability or uncertainty of the feed properties, the process results are compromised. Another expensive iteration is repeated wherein the conveyor has to be removed from the bowl-conveyor-gear/backdrive assembly. The existing dip weir or baffle would be replaced with another one of a different size to provide a different gap height prior to reassembling the machine. Not only is there a cost issue, there is also time loss which could be critical. Based on field experience, the optimal gap changes with needs. The driest cake at a moderate throughput rate requires the smallest gap, whereas moderately dry cake at the highest throughput rate demands a wider gap. This need may vary with time and circumstances, rendering it difficult to predetermine an universally optimal gap.
An object of the present invention is to provide another option for controlling cake moisture content, which could be possibly less expensive and more flexible than previously proposed systems. The problem is to provide a cake flow control structure which allows for adjustability, to accommodate cakes of different compositions and rheological behavior. The adjustability should preferably be finely controllable and easily accessible for adjustment or repair.